This invention relates to fluorescent lamp ballast circuits and, more particularly, to a simple, compact, economical ballast circuit which has excellent power regulation under varying conditions of line voltage.
Most fluorescent lamp ballasting circuits are of one of three general types, namely, a lamp preheat type magnetic circuit, a lamp rapid start magnetic circuit, or a lamp instant start magnetic circuit. In preheat circuits, closing of the on-off switch permits a heating current to flow through the electrode at each end of the lamp and when a starter switch is opened, a high-voltage pulse is impressed across the electrodes which initiates the discharge between the electrodes. The starting circuit may be controlled by a munually operated switch or by an automatic make-and-break type of switch, which normally takes the form of a so-called glow starter. Once the discharge is initiated within the lamp, the current therethrough is controlled by a series-connected inductor or so-called choke.
In so-called rapid start circuits, the lamp electrodes are preheated for a short time prior to initiating the discharge and most such circuits are of the two-lamp series-sequence type in which the lamps start is sequence and thereafter operate in series with a current-limiting inductor.
In instant-start circuits, the discharge is initiated without any preheating of the electrodes by the application of a sufficiently high voltage. Such circuits can be of the lead-lag type or series-sequence type and once the discharge is initiated, the current through the lamps is limited by a ballasting inductor. All of the foregoing circuits are very well known and are extensively used.
It has been known for many years that fluorescent lamps can be ballasted by a fixed value resistor and a recent adaptation of such a ballasting arrangement is disclosed in U.S. Pat. No. 3,996,493 dated Dec. 7, 1976. An earlier adaptation of a wire-type resistance ballast having a positive temperature coefficient of resistance is disclosed in U.S. Pat. No. 2,373,219 dated Apr. 10, 1945 wherein the PTC wire resistance is used to control the potential applied to the starting electrodes of a specific fluorescent discharge device.
Material which display a positive temperature coefficient of resistance are well known, such as described in U.S. Pat. No. 3,044,968 dated July 17, 1963, and U.S. Pat. No. 2,976,505 dated Mar. 21, 1961.
In U.S. Pat. No. 3,271,622 dated Sept. 6, 1966 is disclosed a piezoelectric ballast apparatus for fluorescent lamps, and one suggested material for such a ballast is barium titanate, which is an excellent PTC material, see column 2, line 49 of this patent. It has also been suggested to control the power input to a fluorescent lamp by placing a barium titanate (PTC) resistor in series with a choke, as described in Japanese Utility Model No. 44-7972 dated Mar. 3, 1969. PTC resistors to replace glow switch starters in fluorescent lamps are described in British Pat. No. 1.072,717 dated June 17, 1967.